Numerous applications, ranging from industrial processes to wastewater management, require alteration or control of gaseous byproducts or dissolved constituents in aqueous mixtures. For example, water treatment frequently involves removal ionizing gases such as ammonia or hydrogen sulfide. This may be accomplished by air stripping, i.e., exposing the liquid to large volumes of air to create non-equilibrium conditions that result in the evolution of the unwanted gases. This practice can be self-defeating if the air itself contains one or more of the constituents sought to be removed, or when the dissolved gas or a bulk liquid component reacts with airborne oxygen or carbon dioxide. Moreover, the bulk solution conditions may complicate removal of gas due to ionization in solution.
This occurs, for example, in cases where the solution must be made basic to enhance the partial pressure of the unwanted dissolved gas. When removing ammonia from a liquid stream, general practice is to elevate the pH of the feed solution to at least 9 (and typically to 11) to shift the form of the dissolved ammonia from ammonium ion, NH.sub.4.sup.+, to free ammonia, NH.sub.3. But the carbon dioxide content of the air used to strip the dissolved free ammonia itself reacts with the basic solution, imparting acidity that results in the need for additional base if complete ammonia removal is to be achieved.
Essentially the converse is true in the removal of H.sub.2 S from aqueous solution. At neutral pH values, hydrogen sulfide is ionized in solution as monohydrogen sulfide, HS.sup.-. Air stripping under these conditions will remove 5-25% of the dissolved sulfur species, representing the amount of the unionized species in equilibrium at neutral pH. However, the sulfide ion in solution reacts rapidly with oxygen to generate disulfide and higher sulfur species that are not gases at normal temperatures and conditions. The more air that is used, the more oxidation will take place, and the less total sulfur that will be removed from solution.
Consequently, air stripping and similar processes that utilize atmospheric exposure can never reduce the concentrations of certain reactive constituents below a threshold level due to impurities in the air itself. These impurities can react with the liquid to be treated to oppose the very process used to effect treatment. In the cases of ammonia or hydrogen sulfide, the stripping air may be free of the gas to be removed and the partial pressure of the gaseous impurity essentially driven to zero, but only with large volumes of air and measures that achieve very high degrees of liquid-gas contact. More obvious limitations arise when the impurities sought to be removed (e.g., CO.sub.2) are themselves present in the air used for stripping, which thereby imposes a floor on the amount of the impurity that may be removed. Unless the inherent content of CO.sub.2 is first removed from the stripping air, the carbon dioxide content of the solution to be treated cannot be reduced below the partial pressure equilibrium point of the carbon dioxide in the air.